1. Field of the Invention
This invention relates generally to the field of digital communications. More particularly, this invention relates to the field of digital communications having requirements for derivation of synchronization from the digital signal and the problems associated with maintaining synchronization in such a system during period of long strings of zeros or low ones density.
2. Background
To meet CCITT Recommendation G.703, (and Bell Publication 62411), T1 communication devices are required to send no more than 15 consecutive zeros in their data stream and must have a minimum ones pulse density of 12.5%. This requirement allows the telephone company's equipment to maintain synchronization. Long strings of consecutive zeros can cause many problems. For example, such long strings of zeros can cause T1 repeater equipment to lose synchronization. A scheme for limiting the length of strings of consecutive zeros is therefore necessary for present digital networks. Among the most important factors in a zero limitation scheme are the use of a minimum of bandwidth as overhead, and keeping the hardware and/or software implementation simple.
In today's networks, 1 out of 8 bit stealing is a method in use that prevents the transmitting of more than 15 consecutive zeros in a data stream. This method is performed simply by setting 1 bit out of every byte, thereby using one-eighth of the users bandwidth. It is the among the simplest available schemes for consecutive zero limitation, but the high overhead seriously impacts data throughput making it undesirable.
Another method of zero limitation involves detecting 15 consecutive zeros, and when there are 15 consecutive zeros the next bit must always be set. Some of the users bandwidth can be reserved to hold the actual 16th bit when this happens. The number of bits reserved for this purpose will give the number of occurrences of 15 consecutive zeros that can be handled without causing a bit error. Errors may still occur using this method if the overhead is less than 6.25%. To apply this method, scramblers, parallel to serial converters, and serial to parallel converters are needed to guarantee a ones density of at least 12.5%.
In September 1981, AT&T announced a method termed Bipolar Eight Zero Substitutions (B8ZS) which provides a clear channel for the primary rate. B8ZS inserts deliberate bipolar violations to permit transmission of zero bytes while providing sufficient pulse density For the repeater equipment and T1 source/sink devices.
ZBTSI, which stands For Zero Byte Time Slot Interchange, is an alternate solution to the problem. It appeared in a contribution to the CCITT standards committee T1X1.4 as a candidate to become a standard. It requires only a small amount of overhead (1 bit For a 127 byte frame), and it requires a smaller investment, from a network viewpoint, than B8ZS. A detailed comparison of ZBTSI and B8ZS is found in G. J. Beveridge et al, "Line Code Formats for ISDN Clear Channels: Stand Alone vs. Integrated Network Solutions", Document No. T1X1.4/85 CB, July 26, 1985.
The present invention, termed the Zero Byte Address Linked List method (ZBALL) is a modification of ZBTSI, that has the same advantages of ZBTSI over B8ZS. ZBALL makes some improvements over ZBTSI in the following respects. ZBALL can use the full 128 addresses available in a byte-wide system, ZBTSI can have a maximum of only 127 bytes per frame. In some cases, For example X.25, a 128 byte (1024 bit) packet size becomes important. ZBTSI may be much more difficult than the present invention to implement in hardware since it involves shifting the location of the bytes in a frame. There is no shifting of bytes in ZBALL, just replacement of the byte's contents.